Life Begets Life

This can be a painful grind; walking up to plants/trees and tapping to harvest.

But the quickest way is with the Eternal Harvest power (obtainable from Temple Lambda) and either in a Greenhouse in an outpost you make - or during the Freestar Rangers mission "Deputized" you'll have to head to a farm on a moon near Akila (Cheyenne system) with Emma Wilcox; on that farm is an outdoor greenhouse with a circle of maize plants; just run around the circle harvesting them and hit Eternal Harvest after each rotation. The whole 500 can be done in 20-30 minutes this way, and you can return at any time so there's no hurry.